Trigger of the Highly Resistive Layer Formation at the Cathode–Electrolyte Interface in All-Solid-State Lithium Batteries Using a Garnet-Type Lithium-Ion Conductor

Interfacial materials design is critical in the development of all-solid-state lithium batteries. We must develop an electrode–electrolyte interface with low resistance and effectively utilize the energy stored in the battery system. Here, we investigated the highly resistive layer formation process at the interface of a layered cathode: LiCoO 2, and a garnet-type solid-state electrolyte: Li 6.4 La 3 Zr 1.4 Ta 0.6 O 12, during the cosintering process using in situ/ex situ high-temperature X-ray diffraction. The onset temperature of the reaction between a lithium-deficient Li x CoO 2 and Li 6.4 La 3 Zr 1.4 Ta 0.6 O 12 is 60 °C, while a stoichiometric LiCoO 2 does not show any reaction up to 900 °C. The chemical potential gap of lithium first triggers the lithium migration from the garnet phase to the Li x CoO 2 below 200 °C. The lithium-extracted garnet gradually decomposes around 200 °C and mostly disappears at 500 °C. Since the interdiffusion of the transition metal is not observed below 500 °C, the early-stage reaction product is the decomposed lithium-deficient garnet phase. Electrochemical impedance spectroscopy results showed that the highly resistive layer is formed even below 200 °C. The present work offers that the origin of the highly resistive layer formation is triggered by lithium migration at the solid–solid interface and decomposition of the lithium-deficient garnet phase. We must prevent spontaneous lithium migration at the cathode–electrolyte interface to avoid a highly resistive layer formation. Our results show that the lithium chemical potential gap should be the critical parameter for designing an ideal solid–solid interface for all-solid-state battery applications.